1. Field of the Invention
The present invention is directed to intraocular lenses, and more particularly to accommodating intraocular lenses.
2. Description of the Related Art
A human eye can suffer diseases that impair a patient's vision. For instance, a cataract may increase the opacity of the lens, causing blindness. To restore the patient's vision, the diseased lens may be surgically removed and replaced with an artificial lens, known as an intraocular lens, or IOL. An IOL may also be used for presbyopic lens exchange.
The simplest IOLs have a single focal length, or, equivalently, a single power. Unlike the eye's natural lens, which can adjust its focal length within a particular range in a process known as accommodation, these single focal length IOLs cannot accommodate. As a result, objects at a particular position away from the eye appear in focus, while objects at an increasing distance away from that position appear increasingly blurred.
An improvement over the single focal length IOLs is an accommodating IOL, which can adjust its power within a particular range. As a result, the patient can clearly focus on objects in a range of distances away from the eye, rather than at a single distance. This ability to accommodate is of tremendous benefit for the patient, and more closely approximates the patient's natural vision than a single focal length IOL.
When the eye focuses on a relatively distant object, the lens power is at the low end of the accommodation range, which may be referred to as the “far” power. When the eye focuses on a relatively close object, the lens power is at the high end of the accommodation range, which may be referred to as the “near” power. The accomomodation range itself is defined as the near power minus the far power. In general, an accommodation range of 4 diopters is considered sufficient for most patients.
The human eye contains a structure known as the capsular bag, which surrounds the natural lens. The capsular bag is transparent, and serves to hold the lens. In the natural eye, accommodation is initiated by a series of zonular fibers, also known as zonules. The zonules are located in a relatively thick band mostly around the equator of the lens, and impart a largely radial force to the capsular bag that can alter the shape and/or the location of the natural lens and thereby change its power.
In a typical surgery in which the natural lens is removed from the eye, the lens material is typically broken up and vacuumed out of the eye, but the capsular bag is left intact. The remaining capsular bag is extremely useful for an accommodating intraocular lens, in that the eye's natural accommodation is initiated at least in part by the zonules through the capsular bag. The capsular bag may be used to house an accommodating IOL, which in turn can change shape and/or shift in some manner to affect the power and/or the axial location of the image.
The IOL has an optic, which refracts light that passes through it and forms an image on the retina, and a haptic, which is a structure that mechanically couples the optic to the capsular bag. During accommodation, the zonules exert a force on the capsular bag, which in turn exerts a force on the optic. The force may be transmitted from the capsular bag directly to the optic, or from the capsular bag through the haptic to the optic.
A desirable optic for an accommodating IOL is one that distorts in response to a squeezing or expanding radial force applied to the equator of the optic (i.e., by pushing or pulling on the edge of the optic, circumferentially around the optic axis). Under the influence of a squeezing force, the optic bulges slightly in the axial direction, producing more steeply curved anterior and/or posterior faces, and producing an increase in the power of the optic. Likewise, an expanding radial force produces a decrease in the optic power by flattening the optic. This change in power is accomplished in a manner similar to that of the natural eye and is well adapted to accommodation. Furthermore, this method of changing the lens power reduces any undesirable pressures exerted on some of the structures in the eye.
One challenge in implementing such an optic is designing a suitable haptic to couple the optic to the capsular bag. The haptic should permit diametric (or, equivalently, radial) motion of the optic by coupling the force exerted by the capsular bag to the edge of the optic, which is generally not available with known haptics.
For instance, consider a haptic disclosed in United States Patent Application Publication No. US 2005/0131535, published Jun. 16, 2005 to Randall Woods. The haptic of Woods is a generally resilient, discoid-shaped body that fits inside the capsular bag. The posterior side of the haptic houses a lens that is typically kept in contact with the posterior wall of the capsular bag. During accommodation, so that the eye can focus on near objects, the capsular bag compresses radially and bulges axially, and the optic, which is held in contact with the posterior wall of the capsular bag, distorts and thereby changes its power.
In Woods, the optic itself is surrounded diametrically by a solid body, referred to by Woods as a “resilient body”. Such a resilient body limits the coupling of radial forces to the optic itself. For instance, a compressive radial force exerted by the capsular bag on the haptic would be partially absorbed by the haptic itself, which would couple the compressive force partially into compressing its own material in the region surround the optic. Likewise, an expansive force exerted by the capsular bag on the haptic would also be largely absorbed by the haptic, which would have to stretch in a tangential manner in the region surrounding the optic.
Haptic designs that are used with other IOL styles are generally unsuitable as well. As an example, consider the haptics used in a style of accommodating IOL in which the forces applied by the capsular bag result primarily in an axial translation of the optic, without a significant distortion of the optic. As a specific example, U.S. Pat. No. 5,275,623, issued Jan. 4, 1994 to Faezeh Sarfarazi, discloses a lens that uses a pair of optics and a flexible haptic that responds to the forces of the capsular bag by changing the separation between the optics. Both optics have individual powers, and the total optical power of the eye is adjusted by altering the separation between the optics. The optics themselves are not significantly distorted. Indeed, the haptic largely prevents distortion of the optics by encircling the circumference of the optic with a ring that axially translates the optic under the influence of the capsular bag forces, but retains its own shape under said forces. In general, a haptic in which the optic is encircled by a thick supporting ring is unsuitable, because the forces imparted by the capsular bag are not sufficiently coupled to the edge of the optic.
As yet another example that known haptics are generally unsuitable for coupling the capsular bag forces directly to the diameter of the optic, consider United States Patent Application Publication No. US 2004/0181279, published Sep. 16, 2004 to Yehoshua Nun. Nun discloses a lens that has an optic located between a rigid haptic and the posterior wall of the capsular bag. As the zonules exert force on the capsular bag, the optic is compressed or expanded against the rigid haptic, leading to a change in the radii of the optic as well as the optic thickness. A potential drawback of the Nun IOL is that the haptic is biased anteriorly by pressure from the optic, and may lead to fatigue at the points where the haptic is mounted against the ciliary body in the eye. In addition, distortion of the optic is accomplished by longitudinal (or, equivalently, axial) forces, exerted between the capsular bag wall and the rigid haptic. Indeed, this haptic is also unsuitable for coupling the capsular bag forces to the edge of the optic.
Accordingly, there exists a need for an intraocular lens with a haptic that permits diametric motion of the optic by efficiently coupling the force exerted by the capsular bag and zonules to the edge of the optic. Such a haptic would enable use of a desirable style of optic, which changes its power (i.e., radius of curvature, shape and/or thickness) in response to a squeezing or expanding force applied radially to its edge.